The invention relates to a method of heat treatment of fine-grained material, such as cement, using a rotary kiln as a combustion zone, a preheating zone for preheating the material with the exhaust gases from the rotary kiln, and a calcination zone arranged between the preheating zone and the combustion zone to which further combustion material is delivered in addition to the exhaust gases from the kiln, tertiary air from a cooling zone being mixed together with the exhaust gases from the kiln immediately after they leave the rotary kiln. The invention also relates to apparatus for carrying out this method.
In processes of the aforesaid type it has proved advantageous from the point of view of planning both the process and the plant to provide a special calcination zone between the rotary kiln and the preheating zone, because in this way the greater part of the calcination treatment of the material can be carried out upstream from the rotary kiln so that the rotary kiln is relieved of strain pyrometrically and can be shorter in length as compared with other known processes in which a substantial proportion of the calcination takes place inside the rotary kiln. However, in shortened rotary kilns numerous problems occur because in the transition region from the rotary kiln to the heat exchanger or to the calcination zone the exhaust gases from the kiln are still at relatively high temperatures so that amongst other things undesirable deposits can build up in this transition region.
A method of the type referred to above has been proposed (DD-PS 51 795) in which the tertiary air coming from a cooler is led directly into the inlet chamber provided at the material inlet end of the rotary kiln so that the exhaust gases escaping from the rotary kiln are mixed together with the cooler tertiary air. However, in this method the cooling of the exhaust gases from the rotary kiln is so drastic that the combustion of the material in the calcination zone is markedly impaired.